I've never made Haggis, although I've got a few recipes for it, but if you want to buy the best ready made try here
http://www.scottishhaggis.co.uk.
Click on the "our products" on the left hand side for a full listing.
The smoked venison haggis is an incredible variation on the classic and their Black Pudding is to die for (but I'm hoping that Franco's mix will sway me on this, I'll find out this weekend).
Incidentally in the USA a traditional Haggis cannot legally be sold as the main ingredient is a sheep's "pluck" which is the heart, liver and lungs. The lungs of a sheep are considered as not being fit for human consumption by the American authorities!!
Tell that to the Scots, I'm sure their history is full of people being poisoned by the sheep's lungs in their national dish!! Rabbie Burns must be turning in his grave.